BIBLIOGRAPHY OF HERSCHEL's WRITINGS. 549 



Herschel, W.: Synopsis of the AVritings of— Contiuued. 



A. D. Vol. p. 



1702 82 11 It does not appear to me that tliore is a sufficient ground for admit- 

 ting the ring of Saturn to be of a very changeable nature. 

 11 Measures of the diameter of the outer ring, reduced to the mean dis- 

 tance of <Sa<«r?i from the earth; 46".832; 47".241 ; 45".803; 

 13 On the rotation of the fifth satellite of Saturn, on its axis. 



The lifth satellite [Japctus} is subject to a change of brightness ; this 

 having been noticed by other observers I did not at iirst pay so 

 much attention to it as it deserved. I afterwards followed this satel- 

 lite with great attention and marked all its changes of apparent 

 brightness. 



13 The result of many observations is as follows: The light of the satel- 



lite is in full splendor during the time it runs through that part of 

 its orbit, which is between 68 and 129 degrees past the inferior con- 

 junction. In this arc it does not fall above one magnitude short of 

 the brightness of ITitau'], 



14 From about 7° past the opposition till towards the inferior conjunc- 



tion it is not only less bright than [^Rhea'j but hardly, if at all, ex- 

 ceeds [_Dione']; or even ITethys^ at elongation. Such a change 

 among the fixed stars and to the naked eye would be from the 2d 

 to the 5th magnitude. 



14 It is now evident that the time of its rotation on its axis cannot dilfer 

 much from the time of its revolution about Saturn ; notwithstand- 

 ing that the light of the satellite has suffered an occasional change 

 of short duration from other causes. 



14 But I may go further and ascertain upon sufiicieut grounds, that this 

 satellite turns once upon its axis exactly in the time it performs 

 one revolution. This degree of accuracy is obtained by taking in 

 the observations of M. Cassixi in Memoires de VAcad. Roy. des Sci- 

 ences, 1705, p. 121 ; (see, also, 1707, p. 96), and those of M. Bernard, 

 op. cit., 1780, p. 378. 



10 Joining all these I conclude that the 5th satellite of Saturn turns upon 

 its axis once in 79 days 7 hours and 47 minutes. 



10 I cannot help reflecting with some i)leasure on the discovery of an 

 analogy which shows that a certain uniform plan is carried on 

 among the secondaries of our solar system; and we may conjecture 

 that probably most of the satellites are governed by the same law, 

 especially if it be founded on such a construction of their figure as 

 makes them more ponderous toward their primary planets. 



17 From the changes in [Jajjetus'i we may conclude that some part of its 

 surface, and this by fiir the largest, reflects much less light than 

 the rest ; and that neither the darkest nor the brightest side of the 

 satellite is turned toward the planet, but partly one and partly the 

 other, though probably less of the bright side. 



17 The great regularity of this change of brightness seems to point out 

 another resemblance of this satellite with our moon. We see the 

 spots on the moon of pretty nearly the same brightness, so as not 

 to be overcast in a very strong degree by dense clouds to disfigure 

 them, and therefore have great reason to surmise that her atmos- 

 phere is extremely rare ; in like manner we may supx^ose the atmos- 

 phere oiJapetus as rare as that of our moon. 



17 On the distance of the fifth satellite. 



Jlauy measures have been made for the purpose of getting the mass. 

 They begin 1791, Sept. 25, and end Oct. 1. 



